DIGESTION 261 



proved to exist not only in yeast, but in certain fruits, being 

 formed there when the fruits are kept in an atmosphere 

 which contains no oxygen. 



The physiological explanation of this observation will 

 be discussed more fully in a subsequent chapter. 



There are other enzymes with a more restricted distri- 

 bution, about whose value to the plant little or nothing is 

 known at present. The cells of a particular microscopic 

 organism, known as Micrococcus urece, decompose urea 

 with the formation of ammonium carbonate, and an enzyme, 

 urease, having the same power, can be extracted from 

 them. Many enzymes can be prepared from bacteria, 

 which set up various changes in proteins, some resulting in 

 the formation of peptone, and others producing toxic sub- 

 stances. Many bacteria excrete a variety of diastase. 



Enzymes of another class do not apparently take any 

 part in digestion, but may be briefly alluded to here. They 

 set up a process of oxidation in the substances they attack, 

 and have consequently been named oxidases. They are 

 apparently very widely distributed, and perform very 

 various functions, being often concerned in bringing about 

 the presence of particular colouring matters. They occur 

 very prominently in Fungi, but are by no means confined 

 to them. Others of a similar character act exactly oppositely 

 and have been called reductases. None of the members 

 of either class have at present been very fully studied 

 from the point of view of their utility to the plants which 

 secrete them. 



The conversion of zymogens into enzymes is much 

 facilitated by a gentle warmth, particularly when a trace of 

 free acid is present. The red rays of light exercise a similar 

 influence in some cases. 



The fermentative activity of protoplasm was alluded to 

 at the opening of this chapter. The living substance of 

 many cells is capable of setting up various fermentative 

 decompositions, apparently identical with those that have 

 been described. Various cells can convert starch into 



